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esource pressure; (4) to investigate whether population production varies with the proportion
of r esistant p lants; ( 5) to d etermine th e v ariation in o r s tability o f th e fitness imp act o n
simulated in sect-resistance i n s usceptible pl ant popul ations of w ild B. j uncea, a s t he
frequency of r esistant pl ants i ncreases. W e us ed l eaf c lipping t o s imulate he rbivory on B.
juncea and to mimic the effect of insect resistance. Simulating herbivory (e.g. defoliation at
the cotyledon stage: Sutherland et al., 2006) allows us to exlude interactions arising from the
interspecific h ybridization c ost a nd e liminate v ariability in herent in n atural in sect at tacks.
Simulated herbivory via leaf clipping and hole-punching (Schooler et al., 2006) is often used
as a substitute for actual herbivory in ecological studies of insect-plant interactions; it o ffers
precise c ontrol on t he amount a nd t iming of pl ant da mage ( Baldwin, 1990) a nd c losely
mimics actual impacts of herbivory (Schooler et al., 2006).
Materials and methods
The 2008 experiment Wild Brassica j uncea (2n=36, A ABB) was obt ained f rom t he
Dijon, INRA l aboratory w eed collection ( ref. X04-021, f rom T urkey). Seeds o f B. j uncea
were sown in Giffy-7 in the greenhouse (22 o C under natural light). Three weeks later, in June
2008, seedlings at the three-leaf stage were transplanted to one of four insect-free cages (3m
width * 6m length * 3m height, protected by a 2mm mesh nylon net) in the INRA common
garden at Dijon. Each cage contained five plots (0.5m * 0.7m), and plots were separated from
each other by 0.6m. In each plot we planted 24 B. juncea seedlings, placed one every 0.1m, in
four rows and six columns. The plots were kept weed-free by hand, and the soil around the
plots was covered by a porous plastic net to prevent weed emergence.
In o rder t o s imulate t he pe rformance of t ransgenic i nsect-resistant p lants in in sectsusceptible
w ild popul ations, w e s imulated i nsect d amage b y cl ipping s ome l eaves o f t he
plants d esignated as s usceptible ( clipped p lants: C P = i nsect-susceptible da maged pl ants),
while other plants remained intact (non-clipped plants: NC = i nsect-resistant healthy plants).
Every second, whole leaf to emerge on the stem of each CP plant was clipped, starting at the
four-leaf s tage a nd c ontinuing t o t he ope ning of t he f irst f lower. T o s imulate t he i nvasion
process of insect-resistant plants into a wild population, five different percentages of NC were
added to plots, with four replicates for each percentage: 0 (T0), 25% (T25), 50% (T50), 75%
(T75), and 100% NC (T100). Plant types (CP or NC) were placed randomly within each plot,
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